Add support for beta releases

[TheAssassin]

It would be great to be able to set some environment variable to force a pre-release to be created instead of a "normal" release. Currently, this is only possible by setting $UPLOADTOOL_PREFIX to some value different from $TRAVIS_TAG, but that is quite counter-intuitive, and also applies some constraints that are not wanted (e.g., a prefix continuous- is added to the name of the release, which is not wanted for beta releases).

I would suggest an UPLOADTOOL_PRERELEASE environment variable to be added. That way, a Pre-release: $TRAVIS_TAG release would be created instead of Release: $TRAVIS_TAG.